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Unlock This CourseImagine weighing all the grass in a field, then all the rabbits that eat the grass, then all the foxes that eat the rabbits. You'd find something fascinating - there's always much more grass than rabbits and more rabbits than foxes! This pattern creates what we call a pyramid of biomass, one of the most important concepts in ecology.
A pyramid of biomass shows us the total mass of living organisms at each feeding level (trophic level) in an ecosystem. It's like a snapshot of who's eating whom and how much living material exists at each level.
Key Definitions:
Biomass gives us the most accurate picture of energy storage in ecosystems. Unlike counting individual organisms, biomass tells us the actual amount of living material available for the next trophic level to consume. A single large tree contains far more biomass than a thousand tiny insects!
Think of a pyramid of biomass like a tower of blocks, where each level gets smaller as you go up. The bottom level (producers) always has the most biomass, whilst the top level (apex predators) has the least. This happens because energy is lost at every step of the food chain.
Every pyramid of biomass follows the same basic pattern, with four main trophic levels. Each level depends on the one below it for energy, but only receives about 10% of the energy from the previous level.
Plants and algae that make their own food through photosynthesis. They form the base of the pyramid with the highest biomass - often 1000-10000 g/m².
Herbivores that eat plants. Their biomass is much lower than producers - typically 100-1000 g/m². Examples include rabbits, deer and caterpillars.
Carnivores that eat herbivores. Even lower biomass - usually 10-100 g/m². Examples include foxes, birds of prey and spiders.
In the African savanna, grass biomass reaches 4000 g/m², supporting zebra and antelope biomass of 400 g/m². Lions and other predators have a biomass of only 40 g/m². This 10:1 ratio is typical of most ecosystems and explains why large predators are so rare compared to their prey.
The pyramid shape isn't random - it's caused by the fundamental laws of energy transfer. When a rabbit eats grass, it doesn't get all the energy stored in that grass. Most of it is lost as heat, used for movement, or passes through as waste.
Understanding energy loss helps explain why biomass pyramids always get smaller at each level. Only about 10% of energy successfully transfers from one trophic level to the next.
Heat Production: About 60% of energy is lost as heat during respiration and movement.
Waste Products: Around 20% passes through as urine and faeces.
Inedible Parts: 10% remains in bones, shells and other parts that aren't eaten.
Scientists measure biomass by collecting samples of organisms and determining their dry mass. This involves removing all water content because water levels can vary dramatically between organisms and seasons.
Measuring biomass requires careful sampling and standardised methods to ensure accurate results across different ecosystems and research studies.
Scientists use quadrats for plants and traps for animals to collect representative samples from the ecosystem without damaging it.
Samples are heated in ovens at 80°C until all water evaporates, leaving only the dry organic matter to be weighed.
Results from small samples are multiplied to estimate the total biomass across the entire ecosystem area.
In a typical British oak woodland, tree biomass averages 15,000 g/m², supporting insect biomass of 150 g/m², bird biomass of 15 g/m² and predator biomass (like weasels) of just 1.5 g/m². This demonstrates the classic 10:1 ratio between trophic levels.
Pyramids of biomass are just one way to represent ecosystems. Scientists also use pyramids of numbers and pyramids of energy, each showing different aspects of how ecosystems function.
Each type of pyramid has advantages and limitations. Understanding when to use each type helps scientists choose the best method for studying different ecosystems.
Numbers: Easy to count but doesn't show energy content.
Biomass: Shows actual living matter but varies seasonally.
Energy: Most accurate for energy flow but hardest to measure.
Whilst most biomass pyramids are pyramid-shaped, some ecosystems create unusual patterns. These exceptions help us understand how different factors affect biomass distribution.
Sometimes biomass pyramids can be inverted, especially in aquatic ecosystems where tiny phytoplankton reproduce very quickly but are constantly eaten by larger zooplankton.
In oceans, phytoplankton biomass might be 40 g/m² whilst zooplankton biomass reaches 100 g/m². This works because phytoplankton reproduce so rapidly that they can support more consumer biomass than their instantaneous biomass suggests.
Global warming is changing biomass pyramids worldwide. Rising temperatures allow some ecosystems to support more biomass, whilst others face reductions due to drought or changing precipitation patterns. Scientists monitor these changes to understand ecosystem health and predict future changes.
Human activities significantly affect biomass distribution in ecosystems. Agriculture, urbanisation and pollution all change the natural balance of biomass pyramids.
Farming creates artificial ecosystems with very different biomass patterns from natural ones. Crop fields have enormous producer biomass but very few consumers.
Wheat fields might have 2000 g/m² of crop biomass but only 20 g/m² of consumer biomass due to pest control.
Grazing animals create simplified pyramids with high herbivore biomass but few predators.
Protected areas maintain natural biomass pyramids that support biodiversity.